Method for production of protein having non-natural type amino acid integrated therein

ABSTRACT

This invention provides a method for producing proteins into which non-natural amino acids have been incorporated at desired positions involving the use of either prokaryote-derived or eukaryote-type aaRS* and prokaryotic cells or eukaryote-type as host cells, the method comprising steps of: (a) introducing genes encoding prokaryote-derived aminoacyl tRNA synthetase mutants having enhanced specificity to non-natural amino acids similar to given natural amino acids (compared with specificity to the natural amino acids) and tRNA genes for non-natural amino acids capable of binding to the non-natural amino acids in the presence of the prokaryote-derived aminoacyl tRNA synthetase mutants into prokaryotic cells that express genes encoding eukaryote-type aminoacyl tRNA synthetase having specificity to the given natural amino acids and tRNA genes capable of binding to the natural amino acids in the presence of eukaryote-type aminoacyl tRNA synthetase; and (b) knocking out genes encoding aminoacyl tRNA synthetase having specificity to the natural amino acids, which are inherent in the prokaryotic cells, and inherent tRNA genes capable of binding to the natural amino acids in the presence of the inherent aminoacyl tRNA synthetase.

TECHNICAL FIELD

The present invention relates to a method for producing proteins into which non-natural amino acids have been incorporated at desired positions, host cells used for such method, and a reagent kit for cell-free protein synthesis used for such method.

BACKGROUND ART

The natural proteins are made up of naturally-occurring 20 amino acid species (hereafter referred to as “natural amino acids”). When protein structures or functions are analyzed or chemical behavior thereof is extended, amino acids that are not present in nature (hereafter referred to as “non-natural amino acids”) may be incorporated into desired positions of an amino acid sequence. Proteins into which non-natural amino acids have been incorporated are referred to as “alloproteins.”

Aminoacyl tRNA synthetase (hereafter referred to as “aaRS”) is an enzyme that binds a given amino acid specifically to given tRNA. Except for certain exceptional instances, 20 different types of such enzymes exist corresponding to each of 20 natural amino acid species. When alloproteins are to be synthesized, new aaRS corresponding to non-natural amino acids (hereafter referred to as “aaRS*”) and tRNA paired with a codon that does not encode natural amino acids (hereafter referred to as “tRNA*”) need to be incorporated into host cells to make them properly function therein. That is, tRNA* to which non-natural amino acids have been bound with the aid of aaRS* can be paired with a codon that does not naturally encode natural amino acids in host cells, in order to synthesize alloproteins into which non-natural amino acids have been incorporated.

In such a case, aaRS* is prepared based on existing aaRS that is specific for a given natural amino acid by modifying functions thereof so as to have activity of recognizing a non-natural amino acid similar to the given natural amino acid as a substrate. When aaRS* that is specific for O-methyltyrosine (i.e., a non-natural amino acid) similar to tyrosine (i.e., a natural amino acid) is to be prepared, for example, TyrRS mutant having enhanced specificity to o-methyltyrosine is prepared based on existing tyrosyl-tRNA synthetase (TyrRS). When alloproteins are synthesized with the use of such aaRS*, use of aaRS* that does not react with 20 natural amino acid species inherent in the host cells and tRNAs corresponding thereto but reacts specifically with given non-natural amino acid and tRNA* is necessary.

Thus, aaRS* having specificity to given non-natural amino acids, which is satisfactorily enhanced compared with specificity to existing natural amino acids, is used. This is because proteins into which natural amino acids have been introduced at sites into which given non-natural amino acids are to be introduced would be disadvantageously synthesized, otherwise. If aaRS* would react with tRNA that is inherent in the host cell besides tRNA*, non-natural amino acids would be introduced into sites into which natural amino acids should be introduced, besides sites into which non-natural amino acids are to be introduced. In order to avoid such problem, when prokaryotic cells are used as host cells, aaRS* that was constructed based on eukaryote-type aaRS may be used, because eukaryote-type aaRS is less likely to react with prokaryotic tRNA. The term “eukaryote-type aaRS” used herein refers to aaRS derived from eukaryotic organisms or aaRS derived from archaebacteria. If prokaryotic cells are used as host cells and prokaryote-derived aaRS* are introduced therein, such aaRS* may disadvantageously synthesize a plurality of types of aminoacyl tRNAs by recognizing tRNAs corresponding to natural amino acids inherent in the host cells as substrates, in addition to tRNA*. In such a case, unambiguous translation of a gene into a protein becomes difficult because of the aforementioned reasons. When prokaryotic host cells are used, accordingly, eukaryote-type aaRS* are to be used. When eukaryote-type cells are used as host cells, aaRS* prepared based on prokaryote-derived aaRS are used.

When alloproteins are synthesized, accordingly, adequate aaRS* needs to be prepared depending on whether the host cells to be used are eukaryotic or prokaryotic cells. aaRS* that can be used regardless of whether the host cells are eukaryote-type or prokaryotic cells rarely exists. When synthesis of alloproteins into which given non-natural amino acids have been incorporated is intended in eukaryote-type and prokaryotic cells, accordingly, preparation of prokaryote-derived aaRS* and eukaryote-type aaRS* is necessary. Preparation of aaRS*, however, requires modification of existing aaRS functions so as to realize activity of recognizing non-natural amino acids as substrates, which disadvantageously necessitates a large amount of labor.

Patent Document 1: WO 2003/014354

Patent Document 2: WO 2004/039989

DISCLOSURE OF THE INVENTION

Under the above circumstances, the present invention is intended to provide a method for producing alloproteins, which involves the use of either prokaryote-derived aaRS* or eukaryote-type aaRS* and which can use prokaryotic cells and eukaryote-type cells as host cells.

The method for producing alloproteins according to the present invention that has attained the above object comprises the following steps of:

(a) introducing genes encoding prokaryote-derived aminoacyl tRNA synthetase mutants having enhanced specificity to non-natural amino acids similar to given natural amino acids, compared with specificity to the natural amino acids, and tRNA genes for non-natural amino acids capable of binding to the non-natural amino acids in the presence of the prokaryote-derived aminoacyl tRNA synthetase mutants into prokaryotic cells that express genes encoding eukaryote-type aminoacyl tRNA synthetase having specificity to the given natural amino acids and tRNA genes capable of binding to the natural amino acids in the presence of the eukaryote-type aminoacyl tRNA synthetase;

(b) knocking out genes encoding aminoacyl tRNA synthetase having specificity to the natural amino acids, which are inherent in the prokaryotic cells, and inherent tRNA genes capable of binding to the natural amino acids in the presence of the inherent aminoacyl tRNA synthetase; and

(c) expressing target proteins that are encoded by target genes having codons paired with anticodons of the tRNA genes for the non-natural amino acids in the prokaryotic cells.

According to the method for producing alloproteins of the present invention, the non-natural amino acids can be incorporated into codons paired with the anticodons to produce desired alloproteins in prokaryotic cells. Prokaryote-derived aminoacyl tRNA synthetase mutants that are used in the present invention are not limited to systems that synthesize alloproteins in prokaryotic cells. Such mutants can be applied to systems that synthesize alloproteins in eukaryotic cells.

Also, the method for producing alloproteins according to the present invention is not limited to systems involving the use of prokaryotic cells as host cells. Such method can be applied to systems involving the use of eukaryote-type aminoacyl tRNA synthetase mutants and eukaryote-type host cells.

The prokaryotic cells according to the present invention have the following properties:

(a) genes encoding eukaryote-type aminoacyl tRNA synthetase having specificity to given natural amino acids and tRNA genes capable of binding to the natural amino acids in the presence of the eukaryote-type aminoacyl tRNA synthetase have been introduced; and

(b) genes encoding aminoacyl tRNA synthetase having specificity to the natural amino acids, which are inherent in the prokaryotic cells, and inherent tRNA genes capable of binding to the natural amino acids in the presence of the inherent aminoacyl tRNA synthetase have been knocked out.

The prokaryotic cells according to the present invention having such properties would use eukaryote-type aminoacyl tRNA synthetase and corresponding eukaryote-type tRNA, when incorporating natural amino acids similar to non-natural amino acids.

Further, the reagent kit for cell-free protein synthesis according to the present invention comprises at least the following elements:

(a) prokaryote-derived aminoacyl tRNA synthetase mutants having enhanced specificity to non-natural amino acids similar to given natural amino acids (compared with specificity to the natural amino acids);

(b) tRNA for non-natural amino acids capable of binding to the non-natural amino acids in the presence of the prokaryote-derived aminoacyl tRNA synthetase mutants;

(c) an amino acid solution comprising the non-natural amino acids; and

(d) an extract of prokaryotic cells in which genes encoding eukaryote-type aminoacyl tRNA synthetase having specificity to the given natural amino acids and tRNA genes capable of binding to the natural amino acids in the presence of the inherent aminoacyl tRNA synthetase have been introduced and from which genes encoding aminoacyl tRNA synthetase having specificity to the natural amino acids, which are inherent in the prokaryotic cells, and inherent tRNA genes capable of binding to the natural amino acids in the presence of the inherent aminoacyl tRNA synthetase have been knocked out.

When such reagent kit for cell-free protein synthesis is used, eukaryote-type aminoacyl tRNA synthetase and corresponding eukaryote-type tRNA would be used, when incorporating natural amino acids similar to non-natural amino acids. The reagent kit for cell-free protein synthesis according to the present invention is not limited to systems involving the use of the aforementioned extract of prokaryotic cells. Such kit may be applied to systems involving the use of eukaryote-type aminoacyl tRNA synthetase mutants and the extract of eukaryotic cells.

This description includes part or all of the contents as disclosed in the description and/or drawings of Japanese Patent Application No. 2005-338402, which is a priority document of the present application.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a photograph showing the results of transforming TOP10[ΔtyrU, ΔtyrS, pTK3] and TOP10 cells with the 2541supF plasmids and culturing the same in chloramphenicol-containing medium and chloramphenicol-free medium.

FIG. 2 shows growth curves attained by introducing E. coli TyrRS mutants into TOP10 [ΔtyrT, ΔtyrU, ΔtyrS, pTK3] and TOP10 [ΔtyrU, ΔtyrS, pTK3] cells and growing the same in bromotyrosine-containing medium and bromotyrosine-free medium.

FIG. 3 is a photograph showing the results of extraction of chromosome DNAs from TOP10 [ΔtyrT, ΔtyrU, ΔtyrS, pTK3] and TOP10 cells, PCR and examination of PCR-amplified fragments via electrophoresis.

BEST MODES FOR CARRYING OUT THE INVENTION

Hereafter, the present invention is described in greater detail.

The term “alloprotein” is defined as a protein into which non-natural amino acids have been incorporated. The method for producing proteins into which non-natural amino acids have been incorporated according to the present invention (hereafter referred to as a “method for producing alloproteins”) involves the use of prokaryote-derived or eukaryote-type aminoacyl tRNA synthetase mutants (hereafter referred to as “aaRS*”). Regardless of a host cell type, i.e., prokaryotic cells, eukaryote-type cells, prokaryote-derived cell-free protein synthesis systems, or eukaryote-type cell-free protein synthesis systems, aaRS* can be applied to a wide variety of host cells.

In the present invention, the term “eukaryote-type” refers to both eukaryotes and archaebacteria. When it is described as “eukaryote-type aminoacyl tRNA synthetase,” accordingly, such term refers to either eukaryote-type aminoacyl tRNA synthetase or archaebacteria-derived aminoacyl tRNA synthetase.

Non-Natural Amino Acids

In the present invention, the term “non-natural amino acids” refers to amino acids having structures different from those of 20 natural amino acid species. Since non-natural amino acids have structures similar to those of natural amino acids, non-natural amino acids are classified as derivatives or analogs of given natural amino acids. Examples of non-natural amino acids include derivatives of tyrosine that are natural amino acids, such as 3-substituted tyrosine and 4-substituted tyrosine. Examples of 3-substituted tyrosine include 3-halogenated tyrosine, such as 3-iodotyrosine and 3-bromotyrosine. Examples of 4-substituted tyrosine include 4-acetyl-L-phenylalanine, 4-benzoyl-L-phenylalanine, 4-azide-L-phenylalanine, O-methyl-tyrosine, and 4-iodo-L-phenylalanine.

Non-natural amino acids are not limited to tyrosine derivatives. Examples of non-natural amino acids include azidoalanine, azidohomoalanine, norleucine, norvaline, 4-aminotryptophan, 7-azatryptophan, 6-methyltryptophan, acetyllysine, ε-Boc-lysine, ε-methyllysine, 1-naphthylalanine, 2-naphthylalanine, styrylalanine, diphenylalanine, thiazolylalanine, 2-pyridylalanine, 3-pyridylalanine, 4-pyridylalanine, anthrylalanine, 2-amino-5-hexynoic acid, furylalanine, benzothienylalanine, thienylalanine, allylglycine, propargylglycine, phosphorylserine, phosphorylthreonine, and 2,3-diaminopropionic acid.

Aminoacyl tRNA Synthetase Mutants

In the present invention, “aaRS*” refers to mutant aminoacyl tRNA synthetase having enhanced specificity to non-natural amino acids similar to given natural amino acids, compared with specificity to the natural amino acids. When specificity is enhanced, an activity value regarding non-natural amino acids (i.e., the value obtained by dividing the reaction rate, K_(cat), by the Michaelis constant, K_(m)) is significantly larger than the activity value regarding natural amino acids. The activity value can be measured by in vitro assay, and a relative activity value can be determined based on genetic data.

aaRS* thus defined can be obtained by introducing a mutation into a given site of known aminoacyl tRNA synthetase corresponding to natural amino acids. Known aminoacyl tRNA synthetase corresponding to natural amino acids first recognizes amino acids specifically, and it is activated with the addition of AMP, at the time of aminoacyl tRNA synthesis. Regarding known aminoacyl tRNA synthetase, a site that contributes to specific amino acid recognition is known, and such specificity can be changed by introducing a mutation into the relevant site. Based on such finding, a mutation that can reduce specificity to natural amino acids and enhance specificity to non-natural amino acids similar to the natural amino acids can be introduced. Thus, introduction of a mutation into a given site of known aminoacyl tRNA synthetase enables preparation of aaRS* having desired specificity.

Such aaRS* may be derived from prokaryotes or eukaryotes. An example of prokaryote-derived aaRS* is aaRS* (referred to as mutant TyrRS) having enhanced specificity to 3-iodo-L-tyrosine (i.e., a non-natural amino acid), compared with specificity to tyrosine (i.e., a natural amino acid). Mutant TyrRS is described in the following document. (Kiga, D., Sakamoto, K., Kodama, K., Kigawa, T., Matsuda, T., Yabuki, T., Shirouzu, M., Harada, Y., Naklayama, H., Takio, K., Hasegawa, Y., Endo, Y., Hirao, I. and Yokoyama, S., 2002, An engineered Escherichia coli tyrosyl-tRNA synthetase for site-specific incorporation of an unnatural amino acid into proteins in eukaryotic translation and its application in a wheat germ cell-free system, Proc. Natl. Acad. Sci. U.S.A., 99, 9715-9723)

According to this document, substitution of sites corresponding to tyrosine (Y) at position 37 and glutamine (Q) at position 195 in E. coli-derived tyrosyl-tRNA synthetase with other amino acid residues enables production of mutants having enhanced specificity to 3-halogenated tyrosine (non-natural amino acids). More preferably, mutants in which a position corresponding to tyrosine (Y) at position 37 is substituted with valine (V), leucine (L), isoleucine (I), or alanine (A) and a position corresponding to glutamine (Q) at position 195 is substituted with alanine (A), cysteine (C), serine (S), or asparagine (N) can be used. Such mutants have particularly enhanced specificity to 3-iodo-L-tyrosine.

Genes encoding such mutants can be easily prepared by known genetic engineering techniques. For example, genes encoding such mutants can be obtained by site-directed mutagenesis or with the use of a commercialized kit for site-directed mutagenesis.

Examples of other aaRS* derived from prokaryotes include those described in Chin, J. W., Cropp, T. A., Anderson, J. C., Mukherji, M., Zhang, Z., and Schlutz, P. G., 2003, An expanded eukaryotic genetic code. Science, 301, 964-967 and those described in Deiters, A., Cropp, T. A., Mukherji, M., Chin, J. W., Anderson, J. C., and Schultz, P. G., 2003, Adding amino acids with novel reactivity to the genetic codes of Saccharomyces cerevisiae. J. Am. Chem. Soc. 125, 11782-11783.

Examples of aaRS* derived from eukaryote-type include those described in Santoro, S. W., Wang, L., Herberich, B., King, D. S., Schultz, P. G.: An efficient system for the evolution of aminoacyl-tRNA synthetase specificity, Nature Biotechnol. 20, 1044-1048, 2002 and those described in Wang, L., Brock, A., Herberich, B., Schultz, P. G.: Expanding the genetic code of Escherichia coli. Science 292, 498-500, 2001.

tRNA Genes For Non-Natural Amino Acids

The term “tRNA genes for non-natural amino acids” refers to genes that encode tRNA, which is recognized by the aforementioned aaRS* and which has the 3′ terminus to which activated non-natural amino acids are transferred. Specifically, such aaRS* has activity of recognizing given non-natural amino acids, synthesizing non-natural amino acids-AMP, and transferring the non-natural amino acids to the 3′ terminus of tRNA for non-natural amino acids.

Here, tRNA for non-natural amino acids has an anticodon that is paired specifically with a genetic code other than the codons corresponding to 20 natural amino acid species. Preferably, an anticodon of tRNA for non-natural amino acids is composed of a sequence paired with a nonsense codon comprising an UAG amber codon, an UAA ochre codon, and an UGA opal codon. In other words, tRNA for non-natural amino acids is preferably nonsense suppressor tRNA. tRNA for non-natural amino acids having an anticodon paired with an UAG (i.e., an amber codon) is particularly preferable for the following reasons. That is, an opal codon may be sometimes translated into tryptophan at low efficiency, and such codon may be disadvantageously translated into two types of amino acids, i.e., non-natural amino acid and tryptophan. Thus, use of an opal codon is not adequate. Another reason is the presence of G as the third position of an amber codon. Nucleotide pairing of the third position of codon with the first position of anticodon is relatively unstable, and stable GC nucleotide pairing at this position is advantageous for suppressor tRNA to efficiently translate an UAG codon into a non-natural amino acid.

When a mutant of prokaryote-derived aaRS is used as the aforementioned aaRS*, the tRNA genes obtained from the same prokaryote can be used as the tRNA genes for non-natural amino acids. When the aforementioned E. coli-derived mutant TyrRS is used, use of the E. coli-derived suppressor tRNA gene is particularly preferable.

An anticodon of tRNA for non-natural amino acids is not limited to a sequence corresponding to a termination codon. Such anticodon may be composed of a sequence paired specifically with a codon comprising four or more nucleotides. Further, an anticodon of tRNA for non-natural amino acids may comprise non-natural nucleotides. In such a case, the other non-natural nucleotide that can form a nucleotide pair specifically with the non-natural nucleotide is introduced into a relevant site of the codon. Examples of pairs of such non-natural nucleotides include a pair of isoguanine and isocytidine and a pair of 2-amino-6-(2-thienyl)purine and pirydin-2-one.

Host Cells

Eukaryote-type or prokaryotic host cells may be used, regardless of whether the aforementioned aaRS* genes and tRNA genes for non-natural amino acids are derived from prokaryotes or of eukaryote-type.

When prokaryote-derived aaRS* genes and tRNA genes for non-natural amino acids are used and prokaryotic host cells are used, genes encoding aaRS having specificity to natural amino acid similar to the non-natural amino acid that serves as substrate for aaRS* (hereafter referred to as “corresponding aaRS gene inherent in prokaryotic cells”) and tRNA genes capable of binding to natural amino acids in the presence of the corresponding aaRS gene inherent in prokaryotic cells (hereafter referred to as “corresponding tRNA genes inherent in prokaryotic cells”) are substituted with eukaryote-type aaRS genes (hereafter referred to as “corresponding eukaryote-type aaRS genes”) and tRNA genes (hereafter referred to as “corresponding eukaryote-type tRNA genes”) in the prokaryotic cells.

When host cells are prokaryotic cells, more specifically, corresponding aaRS genes inherent in prokaryotic cells and corresponding tRNA genes inherent in prokaryotic cells are knocked out in the prokaryotic cells, and corresponding eukaryote-type aaRS genes and corresponding eukaryote-type tRNA genes are introduced into the prokaryotic cells. Prokaryote-derived aaRS* would selectively aminoacylate tRNA for non-natural amino acids without aminoacylating corresponding eukaryote-type tRNA. Also, corresponding eukaryote-type aaRS would selectively aminoacylate corresponding eukaryote-type tRNA without aminoacylating tRNA for non-natural amino acids.

When corresponding eukaryote-type aaRS genes and corresponding eukaryote-type tRNA genes are introduced into prokaryotic cells, expression vectors into which corresponding eukaryote-type aaRS genes and corresponding eukaryote-type tRNA genes have been expressibly introduced may be used, or such corresponding eukaryote-type aaRS genes and corresponding eukaryote-type tRNA genes may be expressibly introduced into the genomes of prokaryotic cells. Since genes that have been introduced can be expressed at high levels and alloproteins can be efficiently synthesized, use of expression vectors is preferable.

In the thus prepared prokaryotic cells, natural amino acids similar to target non-natural amino acids are incorporated into proteins with the aid of eukaryote-type aaRS and tRNA, so that alloproteins having target amino acid sequences can be properly synthesized.

Prokaryotic cells that can be used as host cells herein are not particularly limited. Examples thereof include E. coli and Bacillus subtilis.

When the aforementioned E. coli-derived mutant TyrRS is used, tyrosyl-tRNA synthetase derived from Methanococcus jannaschii can be used as eukaryote-type aaRS. Methanococcus jannaschii-derived tyrosyl-tRNA synthetase is composed of any protein selected from the group consisting of (a), (b), and (c):

(a) a protein comprising the amino acid sequence as shown in SEQ ID NO: 2;

(b) a protein comprising an amino acid sequence derived from the amino acid sequence as shown in SEQ ID NO: 2 by deletion, substitution, or addition of one or several amino acids and having activity of activating tyrosine and synthesizing tyrosyl tRNA; and

(c) a protein encoded by a polynucleotide hybridizing under stringent conditions to a complementary strand of the polynucleotide encoding a protein comprising the amino acid sequence as shown in SEQ ID NO: 2 and having activity of activating tyrosine and synthesizing tyrosyl tRNA.

When the aforementioned E. coli-derived mutant TyrRS is used, Methanococcus jannaschii-derived tyrosine tRNA genes can be used as eukaryote-type tRNAs. Methanococcus jannaschii-derived tyrosine tRNA is composed of any polynucleotide selected from the group consisting of (a), (b), and (c) below:

(a) a polynucleotide consisting of nucleotides 4334 to 4410 of SEQ ID NO: 1;

(b) a polynucleotide comprising a nucleotide sequence derived from the sequence consisting of nucleotides 4334 to 4410 of SEQ ID NO: 1 by deletion, substitution, or addition of one or more nucleotides and capable of binding activated tyrosine in the presence of Methanococcus jannaschii-derived tyrosyl-tRNA synthetase; and

(c) a polynucleotide hybridizing under stringent conditions to a complementary strand of a polynucleotide consisting of nucleotides 4334 to 4410 of SEQ ID NO: 1 and capable of binding activated tyrosine in the presence of Methanococcus jannaschii-derived tyrosyl-tRNA synthetase.

Under stringent conditions, specific hybridization takes place but nonspecific hybridization does not take place. For example, hybridization may be carried out using 6×SSC buffer (0.9M NaCl, 0.09M sodium citrate) and at 55° C.

When prokaryote-derived aaRS* genes and tRNA genes for non-natural amino acids are used and eukaryote-type cells are used as host cells, aaRS specific to natural amino acids inherent in the eukaryote-type cells and tRNA genes capable of binding to natural amino acids in the presence of the aaRS can be used without substitution. In such a case, eukaryote-type host cells can be used remaining unchanged.

Eukaryote-type cells that can be used as host cells are not particularly limited. Examples thereof include eukaryotic cells, such as yeast, plant, insect, and mammalian cells, and archaebacteria cells. Mammalian cells are particularly preferable since the gene recombinant systems thereof have been established. Examples of useful mammalian cells include Chinese hamster ovary (CHO) cells and COS cells. Specific examples include SV40-transformed monkey kidney cells CV1 (COS-7), human embryonic kidney cells (293 cells), Chinese hamster ovary/-DHFR cells, mouse Sertoli cells (TM4), human pneumocytes (W138), human liver cells (Hep G2), and mouse breast cancer cells (MMT060562).

When eukaryote-type aaRS* genes and tRNA genes for non-natural amino acids are used and eukaryote-type cells are used as host cells, genes encoding aaRS having specificity to natural amino acids similar to non-natural amino acids that serve as substrates for aaRS* (hereafter referred to as “corresponding aaRS genes inherent in eukaryotes”) and tRNA genes capable of binding to natural amino acids in the presence of the corresponding aaRS genes inherent in eukaryotes (hereafter referred to as “corresponding tRNA genes inherent in eukaryotes”) are substituted with prokaryote-derived aaRS genes (hereafter referred to as “corresponding prokaryote-derived aaRS genes”) and tRNA genes (hereafter referred to as “corresponding prokaryote-derived tRNA genes”) in the eukaryote-type cells.

When host cells are eukaryote-type cells, more specifically, corresponding aaRS genes inherent in eukaryotes and corresponding tRNA genes inherent in eukaryotes are knocked out in the eukaryote-type cells, and corresponding prokaryote-derived aaRS genes and corresponding prokaryote-derived tRNA genes are introduced into the eukaryote-type cells. Eukaryote-type aaRS* would selectively aminoacylate tRNA for non-natural amino acids without aminoacylating corresponding prokaryote-derived tRNA. Also, corresponding prokaryote-derived aaRS would selectively aminoacylate corresponding prokaryote-derived tRNA without aminoacylating tRNA for non-natural amino acids.

When corresponding prokaryote-derived aaRS genes and corresponding prokaryote-derived tRNA genes are introduced into eukaryote-type cells, expression vectors into which corresponding prokaryote-derived aaRS genes and corresponding prokaryote-derived tRNA genes have been expressibly introduced may be used, or such corresponding prokaryote-derived aaRS genes and corresponding prokaryote-derived tRNA genes may be expressibly introduced into the genomes of eukaryote-type cells. Since genes that have been introduced can be expressed at high levels and alloproteins can be efficiently synthesized, use of expression vectors is preferable.

In the thus prepared eukaryote-type cells natural amino acids similar to target non-natural amino acids are incorporated into proteins with the aid of prokaryote-derived aaRS and tRNA, so that alloproteins having target amino acid sequences can be properly synthesized.

When eukaryote-type aaRS* genes and tRNA genes for non-natural amino acids are used and prokaryotic cells are used as host cells, aaRS specific to natural amino acids inherent in the prokaryotic cells and tRNA genes capable of binding to natural amino acids in the presence of the aaRS can be used without substitution. In such a case, prokaryotic host cells can be used remaining unchanged.

Target Protein and Method for Producing the Same

The aforementioned aaRS* genes, tRNA genes for non-natural amino acids, and host cells can be used to prepare target proteins (i.e., alloproteins) into which non-natural amino acids have been incorporated. Target proteins are not particularly limited, provided that sequences thereof comprise codons at desired sites, which are paired with anticodons of the aforementioned tRNA for non-natural amino acids of the genes encoding the target proteins. Thus, alloproteins having non-natural amino acids at desired sites can be prepared. Specifically, desired sites of wild-type genes may be mutated into sequences that are paired with anticodons of tRNA for non-natural amino acids via site-directed mutagenesis to prepare genes encoding alloproteins.

The resulting genes encoding the target proteins are introduced into host cells by a conventional technique and expressed therein. In host cells, alloproteins into which non-natural amino acids have been incorporated at sites of codons paired with anticodons of tRNA for non-natural amino acids of the above genes can be synthesized.

Target proteins (i.e., alloproteins) are not particularly limited. Examples thereof include a group of proteins associated with cell signaling (e.g., epidermal growth factor receptors, nerve growth factor receptors, Grab2 proteins, Src kinase, and Ras proteins), a group of proteins associated with translation (e.g., polypeptide elongation factors, initiating factors, transcription termination factors, ribosome proteins, and aminoacyl tRNA synthetase), transcription factors, and membrane proteins.

The prepared alloproteins can be used for (i) structure determination via X-ray crystallographic analysis, (ii) photo-crosslinking or site-directed fluorescent labeling for elucidation of cell signaling pathways, (iii) use as a proteinous drug upon site-directed polyethyleneglycolation for enhancing drug efficacy, and other purposes. According to protein function analysis via site-directed amino acid substitution, amino acids that can be used for substitution are limited to 20 natural amino acid species in the past. Use of non-natural amino acids enables amino acid substitution with a wide variety of amino acid residues without limitation. Thus, analysis of prepared mutants enables elucidation of roles of amino acid residues at specific sites in proteins.

Reagent Kit for Cell-Free Protein Synthesis

aaRS* and tRNA for non-natural amino acids described above can be used as part of a so-called “reagent kit for cell-free protein synthesis.” In general, a reagent kit for cell-free protein synthesis comprises various cell extracts while maintaining the protein synthesis capacities. In cell-free protein synthesis systems, translation systems and transcription/translation systems are known. With the addition of mRNA or DNA encoding a target protein to the reaction solution, proteins are synthesized with the aid of activities of various enzymes contained in extracts.

The reagent kit for cell-free protein synthesis according to the present invention comprises at least an amino acid solution comprising non-natural amino acids similar to given natural amino acids, a set of aaRS* and tRNA for non-natural amino acids described above, and extracts of host cells. The reagent kit for cell-free protein synthesis according to the present invention may further comprise a solution of vector DNA that can incorporate genes encoding target proteins therein and a set of reagents comprising RNA polymerase for transcribing genes encoding target proteins, which are included in general, commercially available, and known reagent kit for cell-free protein synthesis.

In the reagent kit for cell-free protein synthesis according to the present invention, the extracts of host cells may be of either eukaryote-type or prokaryotic cells. When prokaryote-derived aaRS* and an extract of prokaryotic cells are used, however, extracts of prokaryotic cells in which the aforementioned “corresponding aaRS genes inherent in prokaryotic cells” and from which “corresponding tRNA genes inherent in prokaryotic cells” have been knocked out, and the “corresponding eukaryote-type aaRS genes” and “corresponding eukaryote-type tRNA genes” have been introduced should be used. In such a case, “aaRS* genes” and “tRNA genes for non-natural amino acids” may further be introduced into prokaryotic cells, or “aaRS*” and “tRNA for non-natural amino acids” are added to the culture solution of prokaryotic cells remaining unchanged, so as to incorporate “aaRS*” and “tRNA for non-natural amino acids” into the extract of prokaryotic cells.

When prokaryote-derived aaRS* and an extract of eukaryote-type cells are used for the reagent kit for cell-free protein synthesis according to the present invention, also, aaRS specific to natural amino acids inherent in the eukaryote-type cells and tRNA genes capable of binding to natural amino acids in the presence of aaRS can be used without substitution. In such a case, an extract of wild-type eukaryote-type cells can be used remaining unchanged.

When eukaryote-type aaRS* and an extract of eukaryote-type cells are used for the reagent kit for cell-free protein synthesis according to the present invention, an extract of eukaryote-type cells in which the “corresponding aaRS genes inherent in eukaryotes” and from which the “corresponding tRNA genes inherent in eukaryotes” have been knocked out is used. In such a case, the “corresponding prokaryote-derived aaRS genes” and the “corresponding prokaryote-derived tRNA genes” may be introduced into eukaryote-type cells, and the “corresponding prokaryote-derived aaRS” and “corresponding prokaryote-derived tRNA” can thus be incorporated into the extract of eukaryote-type cells. Alternatively, products of “corresponding prokaryote-derived aaRS genes” and “corresponding prokaryote-derived tRNA genes” may be added to extracts.

When eukaryote-type aaRS* and an extract of prokaryotic cells are used for the reagent kit for cell-free protein synthesis according to the present invention, aaRS specific to natural amino acids inherent in the prokaryotic cells and tRNA genes capable of binding to natural amino acids in the presence of the aaRS can be used without substitution. In such a case, extracts of wild-type prokaryotic cells can be used remaining unchanged.

Hereafter, the present invention is described in greater detail with reference to the examples, although the technical scope of the present invention is not limited thereto.

Example 1

In this example, a method for producing, alloproteins wherein E. coli-derived aaRS* (i.e., mutant TyrRS having enhanced specificity to 3-iodo-L-tyrosine compared with specificity to tyrosine) was used, and E. coli host cells were used was examined.

At the outset, mutant TyrRS genes and suppressor tRNA^(Tyr) genes were prepared in the following manner.

Mutant TyrRS

E. coli TyrRS genes were isolated from E. coli chromosomes and then subjected to cloning. The cloned genes were then subjected to amino acid substitution, i.e., from tyrosine to valine (position 37) and from asparagine to cysteine (position 195), so as to prepare mutant TyrRS genes having enhanced specificity to 3-iodo-L-tyrosine compared with specificity to tyrosine. Preparation of E. coli chromosomes, isolation of genes of interest from the chromosomes via a gene amplification technique (i.e., PCR), and cloning of the isolated genes into adequate vectors can be easily performed via known gene engineering techniques. In order to express mutant TyrRS genes, promoters originating from TyrRS genes may be subjected to isolation and cloning together with TyrRS structural genes. A specific example of a vector that can be used is a vector originating from the pBR322 plasmid.

Suppressor tRNA^(Tyr)

A full-length of a gene of small-sized RNA such as tRNA can be prepared via a chemical technique. The nucleotide sequence of the suppressor tRNA^(Tyr) gene is GGTGGGGTTCCCGAGCGGCCAAAGGGAGCAGACTCTAAATCTGCCGTCATCGA CTTCGAAGGTTCGAATCCTTCCCCCACCACCA. A transcription promoter originating from the lpp gene was isolated from the E. coli chromosome and ligated to a site in front of the gene of interest. A transcription termination sequence originating from the rrnC gene that had been prepared via chemical synthesis was ligated to a site behind the gene of interest. The full-length nucleotide sequence after ligation is represented by: GCATGCGGCGCCGCTTCTTTGAGCGAACGATCAAAAATAAGTGGCGCCCCATC AAAAAAATATTCTCAACATAAAAAACTTTGTGTAATACTTGTAACGCTGCCATC AGACGCATTGGTGGGGTTCCCGAGCGGCCAAAGGGAGCAGACTCTAAATCTGC CGTCATCGACTTCGAAGGTTCGAATCCTTCCCCCACCACCATTTATCACAGATTG GAAATTTTTGATCCTTAGCGAAAGCTAAGGATTTTTTTTAGTCGAC. This nucleotide sequence was cloned into a vector originating from pBR322 together with the mutant TyrRS gene.

pEcIYRS Plasmid

The plasmids comprising the mutant TyrRS genes and the suppressor tRNA^(Tyr) genes prepared above were designated as “pEcIYRS.” In E. coli cells into which such plasmids had been introduced, the mutant TyrRS genes are constitutively expressed from the own transcription promoter region, and the suppressor tRNA genes are constitutively expressed from the lpp promoter region.

Construction of “Corresponding Eukaryote Type aaRS” and “Corresponding Eukaryote-Type tRNA” Expression Plasmids

In this example, archaebacteria Methanococcus jannaschii-derived TyrRS and tyrosine tRNA were expressed in host cells as “corresponding eukaryote-type aaRS” and “corresponding eukaryote-type tRNA.” The pTK3 plasmid expressing Methanococcus jannaschii-derived TyrRS genes and tyrosine tRNA genes was constructed in the following manner. The full-length nucleotide sequence of the pTK3 plasmid is shown in SEQ ID NO: 1. The TyrRS gene of archaebacteria Methanococcus jannaschii is composed of the sequence consisting of nucleotides 3284 to 4204 of SEQ ID NO: 1. The amino acid sequence of TyrRS derived from archaebacteria Methanococcus jannaschii is shown in SEQ ID NO: 2. The tyrosine tRNA gene of archaebacteria Methanococcus jannaschii is composed of the sequence consisting of nucleotides 4410 to 4334 in SEQ ID NO: 1. Further, the kanamycin-resistant gene included in the pTK3 plasmid is composed of the sequence consisting of nucleotides 4847 to 5662 in SEQ ID NO: 1. The amino acid sequence of the protein encoded by the kanamycin-resistant gene is shown in SEQ ID NO: 3.

A method for constructing the pTK3 plasmid is summarized as follows. A promoter region of the TrpRS gene was isolated from the E. coli chromosome via PCR and cloned into the pACYC184 vector. The plasmid was designated as pPTRP. The TyrRS gene of Methanococcus jannaschii was isolated from the chromosome thereof via PCR, and cloned into a site immediately behind the TrpRS promoter region of the pPTRP plasmid to prepare the pMjYS plasmid. Preparation of E. coli chromosomes, isolation of genes of interest from the chromosomes via a gene amplification technique (i.e., PCR), and cloning of the isolated genes into adequate plasmids can be easily performed via known gene engineering techniques. The full-length of the tyrosine tRNA gene of Methanococcus jannaschii was prepared via a chemical technique. A transcription promoter originating from the lpp gene was isolated from the E. coli chromosome and ligated to a site in front of the gene of interest. A transcription termination sequence originating from the rrnC gene that had been prepared via chemical synthesis was ligated to a site behind the gene of interest. The resultant was cloned into the pMjYS plasmid to prepare the pMjYRS plasmid. The kanamycin-resistant gene that had been cloned into the commercially available pHSG299 plasmid (Takara Shuzo Co., Ltd.) was prepared, extracted via PCR, and cloned into the pMjYRS plasmid to prepare the pTK3 plasmid.

The thus-obtained pTK3 plasmid was used to transform the E. coli TOP10 cell. The TOP10 cell that has been subjected to treatment which is suitable for electroporation-based transformation was used (commercially available from Invitrogen).

Knockout of “Corresponding Prokaryote-Derived aaRS” and “Corresponding Prokaryote-Derived tRNA”

In this example, tyrosyl-tRNA synthetase genes (tyrS genes) and tyrosine tRNA genes (tyrT genes and tyrU genes) in the E. coli TOP10 host cells were knocked out.

In this example, the Quick and easy BAC modification kit (hereafter abbreviated as “QBM” kit, Gene Bridges) was used to knockout tyrS genes, tyrT genes, and tyrU genes in the TOP10 cells that had been transformed by the pTK3 plasmid. The order for knocking out the genes was not limited to the order described in the examples, provided that the genes would be knocked out in the end. The QBM kit is intended to manipulate bacterial artificial chromosomes (BAC), and it can be used for knocking out the genes in the E. coli chromosomes. The experimental procedure was in accordance with the instructions of the kit. Specifically, two nucleotide sequences each comprising about 50 nucleotides that are located adjacent to the gene x (i.e., the tyrS, tyrT, or tyrU gene, which is to be removed from the chromosome) in the chromosome are designated as x-L and x-R. By amplifying the marker gene via PCR with the use of a pair of primers having x-L and x-R, DNA comprising x-L and x-R ligated to both sides of the marker gene can be obtained. When such DNA is knocked in to the chromosome using the QBM kit, E. coli in which the gene x has been substituted with the marker gene can be obtained.

In view of removal of the marker gene knocked in to the chromosome, use of the chloramphenicol-resistant gene (i.e., the CAT gene) instead of the ampicillin-resistant gene or kanamycin-resistant gene is preferable as the marker gene. In this example, the CAT gene was used as the marker gene.

(1) Knockout of tyrS Gene

In order to knock out the tyrS gene, tyrS-L and tyrS-R were determined as x-L and x-R above.

tyrS-L: (SEQ ID NO: 4) ATGCGTGGAAGATTGATCGTCTTGCACCCTGAAAAGATGCAAAAATCTTG tyrS-R: (SEQ ID NO: 5) ACAGGGAACATGATGAAAAATATTCTCGCTATCCAGTCTCACGTTGTTTA

tyrSL-CmF and tyrSR-CmR1 were designed as a tyrS-L-containing primer and a tyrS-R-containing primer, and these primers were prepared via chemical synthesis.

tyrSL-CmF (70 nucleotides): (SEQ ID NO: 6) ATGCGTGGAAGATTGATCGTCTTGCACCCTGAAAAGATGCAAAAATCTTG ACCCGACGCACTTTGCGCCG tyrSR-CmR1 (71 nucleotides): (SEQ ID NO: 7) TAAACAACGTGAGACTGGATAGCGAGAATATTTTTCATCATGTTCCCTGT TTACGCCCCGCCCTGCCACTC

PCR was carried out using tyrSL-CmF and tyrSR-CmR1 and the pACYC184 vector as a template to amplify a DNA fragment containing the CAT gene comprising tyrS-L and tyrS-R ligated to both ends thereof. SEQ ID NO: 16 shows the nucleotide sequence of the CAT gene. In this example, the CAT gene that comprises a transcription promoter sequence and the Shine-Dalgarno sequence (the SD sequence) but does not comprise a transcription termination sequence was used. The transcription promoter and the SD sequence enable expression of the CAT gene. Since the transcriptional promoter of the pdxY gene located downstream of the tyrS gene is present inside the tyrS gene, a knockout of the tyrS gene would disadvantageously quench pdxY gene expression. By refraining from incorporating a transcription termination sequence into the CAT gene to be introduced, accordingly, the pdxY gene can be coexpressed with the CAT gene.

Subsequently, the thus-amplified DNA fragment was knocked in to the chromosome of the E. coli TOP10 cell using the QBM kit. The CAT gene that had been knocked in to the chromosome would comprise at its both ends tyrS-L and tyrS-R. Accordingly, knock-in of a DNA fragment comprising tyrS-L and tyrS-R directly ligated with each other to the chromosome with the use of the QBM kit enables removal of the CAT gene from the chromosome. If a DNA fragment comprising tyrS-L and tyrS-R directly ligated with each other is properly knocked in to the chromosome, more specifically, the E. coli TOP10 cell becomes sensitive to chloramphenicol (Cm).

The efficiency of proper knock-in with the use of the QBM kit is 1/10,000 or lower. Accordingly, Cm-sensitive E. coli cells were selected in accordance with the following procedure. At the outset, the amplified DNA fragment was introduced, and 1 to 100,000 E. coli cells were cultured in liquid LB medium. When the number of E. coli cells reached 100,000 to 1,000,000 cells/ml via culture, 10 μg/ml Cm was added to the medium, and culture was then continued. Thirty minutes later, 200 μg/ml ampicillin was added to the medium, and culture was continued for about 30 minutes to 1 hour until E. coli cells were lysed. Subsequently, the lysed culture solution was centrifuged, precipitated E. coli cells were inoculated onto an LB plate, and culture was then carried out overnight. One hundred to two hundreds of resulting colonies were selected and Cm-sensitive cells were selected therefrom via the replica method.

Thus, the tyrS genes were knocked out from chromosomes of the E. coli TOP10 cells.

(2) Knockout of tyrT Genes

In the same manner as in “(1) Knockout of tyrS gene” above, the tyrT gene, which is a tyrosine tRNA gene, was knocked out from the E. coli TOP10 cell. In this case, tyrT-L and tyrT-R were determined as x-L and x-R, respectively.

tyrT-L: (SEQ ID NO: 8) AAAATAACTGGTTACCTTTAATCCGTTACGGATGAAAATTACGCAACCAG tyrT-R: (SEQ ID NO: 9) AGTCCCTGAACTTCCCAACGAATCCGCAATTAAATATTCTGCCCATGCGG

tyrTL-CmF and tyrTR-CmR1 were designed as a pair of primers used for PCR involving the use of the CAT gene of the pACYC184 vector as a template and the primers were chemically synthesized.

tyrTL-CmF (70 nucleotides): (SEQ ID NO: 10) AAAATAACTGGTTACCTTTAATCCGTTACGGATGAAAATTACGCAACCAG ACCCGACGCACTTTGCGCCG tyrTR-CmR1 (71 nucleotides): (SEQ ID NO: 11) CCGCATGGGCAGAATATTTAATTGCGGATTCGTTGGGAAGTTCAGGGACT TTACGCCCCGCCCTGCCACTC

In this example, the CAT gene included in the PCR-amplified DNA fragment that comprises a transcription promoter sequence and the Shine-Dalgarno sequence (the SD sequence) but does not comprise a transcription termination sequence was used. The tpr gene located downstream of the tyrT gene can be coexpressed with the CAT gene. With the amplified DNA fragment knocked in to the chromosome of the E. coli TOP10 cells with the use of the QBM kit and the CAT gene then removed in a similar manner, the tyrT gene was successfully knocked out.

(3) Knockout of tyrU Genes

In the same manner as in “(1) Knockout of tyrS gene” above, the tyrU gene, which is a tyrosine tRNA gene, was knocked out from the E. coli TOP10 cell. In this case, tyrU-L and tyrU-R were determined as x-L and x-R, respectively.

tyrU-L: (SEQ ID NO: 12) GTAATCAGTAGGTCACCAGTTCGATTCCGGTAGTCGGCACCATCAAGTCC tyrU-R: (SEQ ID NO: 13) GGCCACGCGATGGCGTAGCCCGAGACGATAAGTTCGCTTACCGGCTCGAA

tyrUL-CmF and tyrUR-CmR1 were designed as a pair of primers used for PCR involving the use of the CAT gene of the pACYC184 vector as a template, and the primers were chemically synthesized.

tyrUL-CmF1 (72 nucleotides): (SEQ ID NO: 14) GTAATCAGTAGGTCACCAGTTCGATTCCGGTAGTCGGCACCATCAAGTCC GATTTTCAGGAGCTAAGGAAGC tyrUR-CmR1 (71 nucleotides): (SEQ ID NO: 15) TTCGAGCCGGTAAGCGAACTTATCGTCTCGGGCTACGCCATCGCGTGGCC TTACGCCCCGCCCTGCCACTC

In this example, the CAT gene included in the PCR-amplified DNA fragment that comprises the SD sequence but does not comprise a transcription termination sequence or a transcription promoter sequence was used. The tyrU genes are cotranscribed with its upstream thrU gene and downstream glyT gene, and a transcription promoter is not necessary at a site upstream of the CAT gene. If the SD sequence for translation is present, expression of the CAT gene becomes possible. By refraining from incorporating a transcription termination sequence into the CAT gene to be introduced, the glyT gene can be coexpressed with the CAT gene. After the amplified DNA fragment was knocked in to the chromosome of the E. coli TOP10 cells with the use of the QBM kit, the tyrU gene was knocked out by removing the CAT gene in a similar manner.

SEQ ID NO: 16 shows the nucleotide sequence of the CAT gene that comprises the transcription promoter sequence and the SD sequence but does not comprise a transcription termination sequence, which was used for knocking out tyrS gene and tyrT gene. SEQ ID NO: 17 shows the nucleotide sequence of the CAT gene that comprises the SD sequence but does not comprise a transcription promoter sequence or transcription termination sequence, which was used for knocking out the tyrU gene.

Preparation of Alloprotein

In the E. coli TOP10 cells, which were obtained by introducing the pTK3 plasmid therein and knocking out the tyrS, tyrT, and tyrU genes in the above-described manner (hereafter referred to as “TOP10*” or “TOP10[ΔtyrT, ΔtyrU, ΔtyrS, pTK3] cells”), tyrosine would be incorporated into a tyrosine-encoding site of genes expressed therein with the aid of archaebacteria Methanococcus jannaschii TyrRS and tyrosine tRNA. Thus, introduction of the pEcIYRS plasmid that expresses mutant TyrRS and suppressor tyrosine tRNA having enhanced specificity to 3-iodo-L-tyrosine compared with specificity to tyrosine into the TOP10* cells would result in synthesis of alloproteins comprising 3-iodotyrosine incorporated into the site of an amber codon paired with an anticodon of suppressor tRNA^(Tyr). Tyrosine would be incorporated into the site of a codon encoding tyrosine in the gene encoding the introduced protein of interest as with the case of other genes, with the aid of archaebacteria Methanococcus jannaschii TyrRS and tyrosine tRNA.

In such a case, mutant TyrRS, which is prokaryote-derived aaRS*, would not aminoacylate Methanococcus jannaschii-derived mRNA, and Methanococcus jannaschii TyrRS would not aminoacylate suppressor tRNA^(Tyr). With the use of the E. coli TOP10 cells into which the pEcIYRS plasmid has been introduced, accordingly, alloproteins comprising non-natural amino acids, i.e., 3-iodotyrosine, incorporated selectively into desired positions can be synthesized. This can be verified by an experiment described below.

This experiment involves the use of an amber mutant gene (SEQ ID NO: 18) of glutathione-S-transferase (hereafter referred to as “GST”) comprising an amber codon introduced into the coding sequence. In this amber mutant gene of GST, the 25th codon from the N terminus is substituted with an amber codon. With the use of a peptide obtained by treating a GST protein with trypsin, a peptide containing such site can be easily detected via mass analysis. This enables identification of whether or not an amino acid at this site is iodotyrosine. The amber mutant gene of GST may be ligated to an adequate expression promoter and cloned into pEcIYRS. Thus, such gene can be expressed in E. coli. An example of an adequate promoter is lacZ-UV5 promoter. The pEcIYRS plasmid into which the amber mutant gene of GST has been cloned is designated as the pEcIYRS-GST(Am) plasmid.

The pEcIYRS-GST(Am) plasmid is introduced into the TOP10* cells in the following manner. TOP10* cells are inoculated into 1.5 ml of liquid LB medium, cultured therein, and allowed to grow to a concentration of about 1,000,000 cells/ml. The culture solution is then recovered, and centrifugation is carried out at 4° C. and 11,000 rpm for 30 seconds to precipitate the cells. The supernatant is discarded, and the precipitate is then suspended in 1 ml of ice-cooled sterilized water. The suspension is subjected to centrifugation again at 4° C. and 11,000 rpm for 30 seconds to precipitate the cells, the supernatant is discarded, and the precipitate is then suspended in 1 ml of ice-cooled sterilized water again. Finally, cells are precipitated via centrifugation at 4° C. and 11,000 rpm for 30 seconds and then suspended in 20 to 30 μl of sterilized water. Thus, TOP10* cells suitable for electroporation are prepared. The pEcIYRS-GST(Am) plasmid is introduced therein by the same manner, so as to prepare TOP10* [pEcIYRS-GST(Am)].

In order to prepare GST into which 3-iodo-L-tyrosine are introduced at position 25, TOP10*[pEcIYRS-GST(Am)] is cultured in liquid LB medium containing 0.1 mg/ml 3-iodo-L-tyrosine and 0.1 mg/ml ampicillin. In order to induce expression of the GST(Am) gene, isopropyl-1-thio-β-D-galactoside (IPTG) is added to a medium to result in a final concentration of 1 mM. The cells are recovered several hours after the addition of IPTG, an E. coli extract is prepared by a known technique, and GST is then purified using a GST affinity column (Amersham). The obtained GST may be analyzed via mass analysis to confirm that iodo tyrosine has been introduced into a predetermined position 25 and that other tyrosine codons have been translated into tyrosine.

Experimentation and Result-1

In accordance with the description above, substituted E. coli cells that can be used as host cells for production of alloproteins involving the use of E. coli-derived mutant TyrRS genes and suppressor tRNA^(Tyr) genes were prepared.

More specifically, the pTK3 plasmid that expresses the Methanococcus jannaschii-derived TyrRS gene and the tyrosine tRNA gene was first transformed into the E. coli TOP10 cell, in accordance with the description above. Thereafter, a mutant was prepared in accordance with the description above by knocking out the tyrS gene in the pTK3-containing E. coli TOP10 cell (TOP10[pTK3] cell) (i.e., TOP10[ΔtyrS, pTK3] cell). Further, a mutant was prepared in accordance with the description above by knocking out the tyrU gene in the mutant (TOP10[ΔtyrS, pTK3] cell) (i.e., TOP10[ΔtyrU, ΔtyrS, pTK3] cell).

An amber suppression test was carried out to verify that the tyrS gene has been knocked out in the resulting TOP10[ΔtyrU, ΔtyrS, pTK3] cell. The 2541supF plasmid was prepared for this test. The amber mutant chloramphenicol-resistant gene and the E. coli tyrosine tRNA-derived amber suppressor tRNA gene have been cloned into the 2541supF plasmid. SEQ ID NO: 19 shows the full-length nucleotide sequence of the 2541supF plasmid.

If the E. coli TyrRS (i.e., the tyrS gene product) is expressed in the E. coli cell into which the 2541supF plasmid has been introduced, suppressor tRNA that is present in the 2541 supF plasmid would function, which in turn would suppress amber mutation and express chloramphenicol resistance. If E. coli TyrRS (i.e., the tyrS gene product) is knocked out in the E. coli cell into which the 2541supF plasmid has been introduced, it is deduced that chloramphenicol resistance would not be expressed. Since suppressor tRNA in the 2541supF plasmid would not be recognized by archaebacteria-derived TyrRS, chloramphenicol resistance would not be expressed even if archaebacteria-derived TyrRS has been expressed in the E. coli cell into which the 2541 supF plasmid has been introduced.

FIG. 1 shows the results of transforming TOP10[ΔtyrU, ΔtyrS, pTK3] and wild-type TOP10 cells with the 2541supF plasmids and then culturing the same in a chloramphenicol-containing medium. As shown in FIG. 1, the TOP10 cell (shown as “WT”) transformed by the 2541 supF plasmid grew on a chloramphenicol (Cm)-containing medium. The TOP10[ΔtyrU, ΔtyrS, pTK3] cells (shown as “tyrS KO”), however, did not grow on a Cm-containing medium. These results indicate that E. coli TyrRS has not been expressed in the TOP10[ΔtyrU, ΔtyrS, pTK3] cells.

Experimentation and Result-2

The tyrT gene was knocked out in the TOP10[ΔtyrU, ΔtyrS, pTK3] cell, in which complete knockout of TyrRS inherent in E. coli had been confirmed, in accordance with the above-described manner to construct mutants (TOP10[ΔtyrT, ΔtyrU, ΔtyrS, pTK3] cells).

Whether or not all tyrosine tRNAs inherent in E. coli had been knocked out in the prepared mutants was examined in the following manner.

Specifically, E. coli TyrRS mutants were first expressed in the TOP10[ΔtyrT, ΔtyrU, ΔtyrS, pTK3] cells. The mutants described in the document (Kiga, D. et al., 2002, Proc. Natl. Acad. Sci. USA 99, 9715-9723) were used. The E. coli TyrRS mutants are capable of recognizing 3-iodotyrosine (or 3-bromotyrosine) and binding the same to E. coli tyrosine tRNA. When E. coli cells in which such mutants have been expressed are inoculated into a bromotyrosine-containing medium, accordingly, bromotyrosine would be incorporated into all proteins instead of tyrosine in such E. coli cells. When E. coli TyrRS mutants function and E. coli tyrosine tRNA are present, accordingly, no protein would function, and E. coli cell cannot grow. When E. coli TyrRS mutants function but E. coli tyrosine tRNA is not present in E. coli, however, E. coli cells can grow on a bromotyrosine-containing medium even if E. coli TyrRS mutants would function.

FIG. 2 shows a growth curve attained by introducing E. coli TyrRS mutants into TOP10[ΔtyrT, ΔtyrU, ΔtyrS, pTK3] and TOP10[ΔtyrU, ΔtyrS, pTK3] cells and growing the same in bromotyrosine-containing medium and bromotyrosine-free medium.

In FIG. 2, cells prepared by introducing E. coli TyrRS mutants into the TOP10[ΔtyrU, ΔtyrS, pTK3] cells are indicated as “ΔtyrS,” and cells prepared by introducing E. coli TyrRS mutants into the TOP10[ΔtyrT, ΔtyrU, ΔtyrS, pTK3] cells are indicated as “tyr-Mj.” In FIG. 2, a horizontal axis shows a culture duration.

As is apparent from FIG. 2, a growth rate began to decrease upon addition of bromotyrosine, and growth stopped 2 hours later in the cells prepared by introducing E. coli TyrRS mutants into the TOP10[ΔtyrU, ΔtyrS, pTK3] cells. This is because E. coli TyrRS mutants function due to the presence of the tyrT gene in the cells prepared by introducing E. coli TyrRS mutants into the TOP10[ΔtyrU, ΔtyrS, pTK3] cells. As is also apparent from FIG. 2, the addition of bromotyrosine did not influence the growth in cells prepared by introducing E. coli TyrRS mutants into the TOP10[ΔtyrT, ΔtyrU, ΔtyrS, pTK3] cells. This indicates that E. coli tyrosine tRNA has been completely knocked out and E. coli TyrRS mutants did not function in cells prepared by introducing E. coli TyrRS mutants into the TOP10[ΔtyrT, ΔtyrU, ΔtyrS, pTK3] cells.

Experimentation and Result-3

Subsequently, whether or not tyrT genes, tyrU genes, and tyrS genes had been completely removed from the genomes of the prepared TOP10[ΔtyrT, ΔtyrU, ΔtyrS, pTK3] cells was examined. Specifically, chromosome DNA was extracted from the TOP10[ΔtyrT, ΔtyrU, ΔtyrS, pTK3] cells, and PCR was carried out using the chromosome DNA as a template.

Chromosome DNA was extracted using the commercialized kit, Dr. GenTLE® (from Yeast) (Takara Bio Inc.). A pair of primers comprising the nucleotide sequences shown below was designed for each of the tyrT gene, the tyrU gene, and the tyrS gene, and the primers were chemically synthesized.

Nucleotide Sequences of Primers for tyrT Gene Amplification

GCAGGACGTTATTCATGTCG (SEQ ID NO: 20) GGGACTTTTGAAAGTGATGG (SEQ ID NO: 21) Nucleotide Sequences of Primers for tyrU Gene Amplification

GTAATCAGTAGGTCACCAGT (SEQ ID NO: 22) TTCGAGCCGGTAAGCGAACT (SEQ ID NO: 23) Nucleotide Sequences of Primers for tyrS Gene Amplification

AAAAGTCGTGTACCGGCAAAG (SEQ ID NO: 24) TAAACAACGTGAGACTGGATAG (SEQ ID NO: 25)

Specifically, primers for tyrT gene amplification were designated as sequences in front of and behind the tyrT and tyrV genes. Primers for tyrU gene amplification were designated as sequences in front of and behind the tyrU gene. Primers for tyrS gene amplification were designated as sequences in front of and behind the tyrS gene.

FIG. 3 shows the results of extraction of chromosome DNAs from TOP10 [ΔtyrT, ΔtyrU, ΔtyrS, pTK3] and wild-type TOP10 cells, PCR and examination of PCR-amplified fragments via electrophoresis. In FIG. 3, the results attained with the use of the TOP10 cells are indicated as “WT,” and the results attained with the use of the TOP10[ΔtyrT, ΔtyrU, ΔtyrS, pTK3] cells are indicated as “ΔtyrU,” “ΔtyrS,” and “ΔtyrT.” In FIG. 3, positions of amplified fragments that appear when amplifying the tyrT gene, the tyrU gene, and the tyrS gene with the use of chromosome DNA of the TOP10 cell as a template are indicated by triangles.

As is apparent from FIG. 3, a fragment to be amplified became shorter when chromosome DNA of the TOP10[ΔtyrT, ΔtyrU, ΔtyrS, pTK3] cells was used, compared with the fragment amplified with the use of chromosome DNA of the TOP10 cells. This demonstrates that the tyrT gene, the tyrU gene, and the tyrS gene had been completely removed from the genomes of the TOP10[ΔtyrT, ΔtyrU, ΔtyrS, pTK3] cells.

It was accordingly demonstrated that TyrRS and tyrosine tRNA inherent in the TOP10[ΔtyrT, ΔtyrU, ΔtyrS, pTK3] cells had been completely knocked out and that such TyrRS and tyrosine tRNA survived via translation of a tyrosine codons by archaebacteria TyrRS and tyrosine tRNA. Such TOP10[ΔtyrT, ΔtyrU, ΔtyrS, pTK3] cells can be used for production of alloproteins involving the use of E. coli-derived mutant TyrRS genes and suppressor tRNA^(Tyr) genes.

Industrial Applicability

According to the method for producing proteins into which non-natural amino acids have been incorporated of the present invention, proteins into which non-natural amino acids have been incorporated at desired positions can be produced with the use of prokaryote-derived aminoacyl tRNA synthetase mutants and prokaryotic cells as host cells. The prokaryote-derived aminoacyl tRNA synthetase mutants can also be used for the method for producing proteins into which non-natural amino acids have been incorporated involving the use of eukaryote-type host cells. According to the present invention, therefore, prokaryote-derived aminoacyl tRNA synthetase mutants can be applied to either systems involving the use of prokaryotic or eukaryotic host cells.

According to the method for producing proteins into which non-natural amino acids have been incorporated, proteins into which non-natural amino acids have been incorporated at desired positions can be produced with the use of eukaryote-type aminoacyl tRNA synthetase mutants and eukaryotic host cells. The eukaryote-type aminoacyl tRNA synthetase mutants can also be applied to the method for producing proteins into which non-natural amino acids have been incorporated involving the use of prokaryotic host cells. According to the present invention, therefore, eukaryote-type aminoacyl tRNA synthetase mutants can be applied to either systems involving the use of prokaryotic or eukaryotic host cells.

All publications, patents, and patent applications cited herein are incorporated herein by reference in their entirety. 

1. A method for producing a target protein into which a non-natural amino acid has been incorporated, wherein said method comprising the steps: (a) transforming a prokaryotic host cell with a mutant gene encoding an E. coli-derived mutant tyrosyl tRS and an E. coli-derived suppressor tyrosine tRNA capable of binding to the non-natural amino acids, wherein said mutant tyrosyl tRS has a higher specificity for the non-natural amino acid compared to specificity for tyrosine and wherein said E. coli host cell further expresses a eukaryote-type tyrosyl-tRNA synthetase and tRNA genes specific for tyrosine; and (b) knocking out the endogenous tyrosyl-tRNA synthetase and tyrosine tRNA genes of the E. coli host cell; and (c) culturing the host cell in a medium thereby expressing the target protein into which a non-natural amino acid is incorporated at the site encoded by the codon to be suppressed.
 2. The method according to claim 1, wherein the eukaryote-type tyrosyl tRNA synthetase is Methanococcus jannaschii-derived tyrosyl-tRNA synthetase.
 3. The method according to claim 2, wherein the Methanococcus jannaschii-derived tyrosyl-tRNA synthetase is protein (a) or (b) below: (a) a protein comprising the amino acid sequence as shown in SEQ ID NO: 2; (b) a protein encoded by a polynucleotide hybridizing under stringent conditions to a complementary strand of the polynucleotide encoding a protein comprising the amino acid sequence as shown in SEQ ID NO: 2 and having activity of activating tyrosine and synthesizing tyrosyl tRNA.
 4. The method according to claim 1, wherein the tRNA genes capable of binding to tyrosine in the presence of the eukaryote-type tyrosyl tRNA synthetase are Methanococcus jannaschii-derived tyrosine tRNA genes.
 5. The method according to claim 4, wherein the Methanococcus jannaschii-derived tyrosine tRNA is polynucleotide (a) or (b) below: (a) a polynucleotide consisting of nucleotides 4334 to 4410 of SEQ ID NO: 1; (b) a polynucleotide hybridizing under stringent conditions to a complementary strand of a polynucleotide consisting of nucleotides 4334 to 4410 of SEQ ID NO: 1 and capable of binding to tyrosine in the presence of the Methanococcus jannaschii-derived tyrosyl-tRNA synthetase.
 6. An E. coli transformed with a eukaryote-type tRNA synthetase and tRNA genes specific for tyrosine (tyrosyl-tRNA synthetase and tyrosyl-tRNA) wherein the endogenous tyrosyl-tRNA synthetase and endogenous tyrosyl-tRNA have been knocked out, wherein the eukaryote-type tyrosyl tRNA synthetase is Methanococcus jannaschii-derived tyrosyl-tRNA synthetase.
 7. The E. coli according to claim 6, wherein the Methanococcus jannaschii-derived tyrosyl-tRNA synthetase is protein (a) or (b) below: (a) a protein comprising the amino acid sequence as shown in SEQ ID NO: 2; (b) a protein encoded by a polynucleotide hybridizing under stringent conditions to a complementary strand of the polynucleotide encoding a protein comprising the amino acid sequence as shown in SEQ ID NO: 2 and having activity of activating tyrosine and synthesizing tyrosyl tRNA.
 8. The E. coli according to claim 6, wherein tRNA genes capable of binding to tyrosine in the presence of the eukaryote-type tyrosyl tRNA synthetase are Methanococcus jannaschii-derived tyrosine tRNA genes.
 9. The E. coli according to claim 8, wherein the Methanococcus jannaschii-derived tyrosine tRNA is polynucleotide (a) or (b) below: (a) a polynucleotide consisting of nucleotides 4334 to 4410 of SEQ ID NO: 1; (c) a polynucleotide hybridizing under stringent conditions to a complementary strand of a polynucleotide consisting of nucleotides 4334 to 4410 of SEQ ID NO: 1 and capable of binding to tyrosine in the presence of the Methanococcus jannaschii-derived tyrosyl-tRNA synthetase.
 10. An extract of the E. coli according to any one of claims 6, 7, 8, or
 9. 